This invention relates to a structure with magnetic properties. In certain applications it would be advantageous if the magnetic permeability of a material could be tailored for that application at least within a specified frequency range. Such a material could have advantages in the design of materials for electromagnetic screening for example.
The invention seeks to provide a structure having a magnetic permeability which is a function of the structure itself even though the constituent parts of the structure do not necessarily of themselves have magnetic properties.
According to the present invention a structure with magnetic properties comprises: an array of capacitive elements, wherein each capacitive element includes a low resistance conducting path and is such that a magnetic component of electromagnetic radiation lying within a predetermined frequency band induces an electrical current to flow around said path and through said associated element and wherein the size of the elements and their spacing apart are selected such as to provide a predetermined permeability in response to said received electromagnetic radiation.
Thus, the present invention provides an artificially structured magnetic material having a permeability, the magnitude and frequency dependence of which can be tailored by appropriate design of the material structure. In the context of this patent, and for the avoidance of doubt, xe2x80x9ccapacitivexe2x80x9d is to be construed as meaning that the electrical impedance is primarily reactive as opposed to resistive and its reactance is such that the induced electrical current leads the voltage.
Natural materials generally exhibit a magnetic permeability xcexc of approximately unity at microwave frequencies, but the magnetic structure of the present invention can provide values of xcexc typically in the range xe2x88x921 to 5 at frequencies in the GHz region, or wider depending on bandwidth.
An important feature of the artificially structured magnetic material of the present invention is the capacitive elements which enable the creation of internal fields that are inhomogeneous, that is on a scale smaller than the wavelength of incoming radiation, and preferably far smaller. These capacitive elements act through the relations
Bav=xcexceffxcexc0Havxe2x80x83xe2x80x83Eq. 1
Dav=∈eff∈0Eavxe2x80x83xe2x80x83Eq. 2
on the average fields to provide effective values for xcexceff and ∈eff which are quite different to those which would be obtained either from the constitutive elements themselves or would be obtained from a simple volume average of material properties. A large variation in the magnetic permeability can be produced by large inhomogeneous electric fields, via a large self capacitance of the array of capacitive elements. The magnetic properties of a structured material in accordance with the invention arises not from any magnetism of its constituent components, but rather from the self capacitance of the elements which interact with the electromagnetic radiation to generate large inhomogeneous electric fields within the structure.
The dimensions of each capacitive element are preferably at least an order of magnitude less than the wavelength of the radiation which it is designed to receive.
Advantageously each capacitive element is of a substantially circular section and in one embodiment comprises two or more concentric conductive cylinders in which each cylinder has a gap running along its length. Each cylinder may be continuous along its length, or can comprise a plurality of stacked planar sections, preferably in the form of split rings, each of which is electrically insulated from adjacent sections. The latter is particularly suited to being fabricated readily using, for example, printed circuit board (PCB) fabrication techniques. Alternatively each element can be in the form of a conductive sheet wound as a spiral. In one embodiment successive turns of the spiral are progressively displaced along the axis of the spiral to form a helical structure, with adjacent turns partially overlapping. Such an arrangement is found to exhibit significant circular bi-refringence. In yet a further embodiment each capacitive element comprises a plurality of stacked planar sections each of which is electrically isolated from each other and is the form of a spiral. Again such a structure can be fabricated readily using PCB manufacturing techniques.
The array can contain elements which are all arranged with their axis in a single direction, e.g. normal to the plane of the array; alternatively the array can contain elements with axis pointing in two or three mutually orthogonal directions. The array can include multiple layers of capacitive elements. The capacitive elements can also take the form of interlocking rings which are electrically insulated or isolated from each other, with each ring having means, eg a gap in it, to prevent circulation of dc currents.
In yet a further embodiment the structure further incorporates a switchable permittivity material enabling the magnetic permeability of the structure to be switched externally by, for example, the application of an external electric field. Advantageously the switchable permittivity material is a ferroelectric material such as barium strontium titanate (BST). The concept of including a switchable permittivity material into such a structure to enable its magnetic properties to be controlled externally is considered to be inventive in its own right.